A problem associated with printed circuit boards, particularly with the thicker, multi-layer boards, is that standard card edge connectors are made with standard spacings between opposed rows of contact elements and the thickness of the circuit board may vary. Accordingly, this variation in spacing between opposed rows of conductive traces or plated fingers along opposite facing edges of the board will produce a variation in the insertion and withdrawal forces of the board edge into the connector and more importantly it effects the reliability with which mating surfaces engage one another.
Another problem associated with circuit boards is that the fingers require plating with noble metals such as gold to prevent corrosion thereof. Gold plating of these fingers is quite expensive in that substantial gold scrap is produced thereby which requires expensive refining techniques to recover. Moreover, gold plating requires additional manufacturing time in making circuit boards and in the event any one of the plated fingers becomes damaged in the manufacturing process, very often the entire board may have to be scrapped.
Connectors having rows of contact elements for mounting to the edge of circuit boards have been developed to obviate the need of plating the traces or fingers on the circuit board. These connectors include a blade extending outwardly from a housing on which are conductive fingers extending from conductive contact elements located in the housing. The connectors are mounted on an edge of the circuit board with leads or pins on the contact elements electrically engaging conductive traces or fingers thereon. Thus, when the blade is inserted into a connecting mating member; e.g. a card edge connector, the traces on the board are electrically connected to traces or circuits on a board or backplane on which the connecting mating member is mounted. However, despite the fact that they eliminate the overall disadvantage of forming plated fingers on the circuit board edge, the prior art edge connectors possess some disadvantages. For example, the fingers generally lay in grooves and are flush with the surface of the blade. When there is lateral shifting between the edge connector and connecting mating member in which the blade is inserted; side to side float due to tolerance mismatch and the like, conductive elements in the mating member will shift off the fingers and onto the blade and break off electrical contact. Another disadvantage with prior art connectors is that the fingers on the blade next to the keying slots are free to shift thereinto so that during insertion they strike the key in the mating member and become damaged. Another disadvantage with prior art connectors is that cross-talk and impedance mismatch between fingers on opposite surfaces of the blade can occur.
It is therefore desirable to provide an edge connector wherein the distal ends of the fingers are necked down and positioned below the surface of the blade to prevent them from being caught and peeled back during insertion but with the rest of the fingers being above the surface of the blade so that at least some of the conductive elements of the mating member will remain engaged therewith during the aforementioned lateral shifting. Further, by reducing the width of the distal ends of the fingers, a barrier between the distal end portion of the fingers and the keying slot prevents the fingers from shifting thereinto. Further, it is desirable to provide an edge connector wherein a ground plane is placed within the blade to minimize cross-talk between fingers and improve impedance/capacitance characteristics. Also, it is desirable to provide an edge connector having sequential contact engaging capabilities.